[selectors, facet] make is_instance and other EClassSelector subclass gather entity classes from 'select' and 'filtered_variable' context information. Closes #1796732 This allow for instance to use the filter form on rqlst where the filtered variable is not selected

[facets] DontUpdateConstraint mecanism introduced a few commit ago isn't needed: we already have the 'needs_update' property controlling this

[facet] more api cleanup: support_and should be a property, so it's easier to deactivate.